The separation of volatile materials by continuous distillation in fractionating towers wherein vapor passes upward countercurrent to liquid flowing downward is and has been widely practiced in the chemical and petroleum industries. Heat is required to operate the process and it is important to recover and reuse as much of this heat as can be done economically.
For instance, referring to an article by Bannon and Marple, published in Chemical Engineering Progress, pp. 41-45, July 1978, (FIGS. 2, 3 and 4) it is seen that apparatus for separating petroleum hydrocarbons into fractions, either as they occur naturally or after they have been modified by cracking, may consist of a fractionating tower with external heat exchangers used to condense vapor leaving the top of the tower, and other external heat exchangers used to cool liquid withdrawn in one or more sidestreams from intermediate points in the tower. Each sidestream, after being cooled, is then recirculated back to the tower. The liquid crude petroleum that is to be separated into fractions is used as coolant in the said heat exchangers, being partially heated thereby, then passed through a furnace or vaporizer where it is partially or wholly vaporized, then passed into the fractionating tower at a point near the bottom. Fuel is burned to supply the required heat to the vaporizer. The more heat the feed stream of crude petroleum can be caused to take up by heat exchange with the vapor leaving the top of the tower and with the liquid withdrawn as sidestreams, the less fuel is required to heat the vaporizer.
Bannon and Marple point out that the conservation of heat is improved by carrying out heat exchange between the liquid feed of crude petroleum and the vapor leaving the top of the tower in two stages rather than in one. They also point out that the recovery of heat is improved by withdrawing, for heat exchange with the feed, several sidestreams rather than only one.
In principle it is possible to recover essentially all the heat required to bring the feed stream of crude petroleum up to its vaporization temperature but in practice the amount of heat recoverable is limited by the capital cost of the heat exchange apparatus including auxiliaries such as pumps, piping and control instruments. Thus it is apparent that, to recover heat economically, it is necessary not only to arrange the apparatus in such a way that the heat exchange takes place in multiple stages but also to minimize the cost of such apparatus.
Accordingly, it is an object of this invention to provide economic methods and apparatus improved over those available in the prior art for the recovery and reuse of heat needed to operate distillation processes.